Electrode, related material, process for production, and use thereof

ABSTRACT

An electrode material is created by forming a thin conformal coating of metal oxide on a highly porous carbon meta-structure. The highly porous carbon meta-structure performs a role in the synthesis of the oxide coating and in providing a three-dimensional, electronically conductive substrate supporting the thin coating of metal oxide. The metal oxide includes one or more metal oxides. The electrode material, a process for producing said electrode material, an electrochemical capacitor and an electrochemical secondary (rechargeable) battery using said electrode material is disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part U.S. application Ser. No.12/015,839, filed on Jan. 17, 2008, and claims priority thereto; theforegoing application being incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a material for use as an electrode forelectrochemical energy storage devices such as electrochemicalcapacitors (ECs) and secondary batteries, and a method for producing thesame. More specifically, this invention relates to an electrode materialconsisting of a highly porous carbon structure coated with a conformalfilm comprising one or more transition metal oxides, a method forproducing the same, and an EC and secondary battery using the electrodematerials.

BACKGROUND OF THE INVENTION

Electrochemical capacitors (referred to as ultracapacitors orsupercapacitors) are devices that use electrolytes that contain andconduct ions in conjunction with electrodes that are ionically andelectronically conductive, as a system to store energy. Unlikebatteries, which store charge within the bulk electrode material itself,ECs store charge at or near the interface between the electrolyte andelectrode material making EC storage a surface phenomenon. EC devicesstore charge via one of two mechanisms: electric “double-layer” orfaradaic “pseudocapacitance”.

Electric double layer capacitors (EDLCs) store charge at the interfacebetween the ion-rich electrolyte and an electronically conductiveelectrode comprising material such as activated carbon (AC). The amountof charge stored is a function of this interfacial area, which is inturn related to the electrode surface area accessible by theelectrolyte-solvated ions. AC electrodes typically have surface areasbetween 1000 and 3000 m²/g achieved by a high concentration ofmicropores (pore diameter<2 nm). This concentration of microporesparadoxically serves to limit the capacitance to a range between 15 and30 uF/cm² because micropores are too small to accommodate the solvatedions necessary for the double layer mechanism. AC electrodes aretypically fabricated as films formed from a paste comprising powderedAC, binder and conductivity-enhancing carbon. While the accessibility ofthe electrolyte to much of the surface area of such an electrode islimited, thus limiting the electrode capacitance and ultimately deviceenergy density, the micropore geometries also serve to limit electrodecharge/discharge rates and ultimately limit device power density.

Pseudocapacitance involves electron transfer in oxidation/reductionreactions that take place between the electrolyte ions and the electrodeactive materials over a voltage range similar to that of the EDLC.Electrode active materials being researched for this type of behaviorinclude graphitic carbons, conductive polymers and transition metaloxides.

Transition metal oxides exhibit characteristics both desirable andotherwise for EC devices. In general, these oxides exhibit improvedspecific capacitance vs. carbon materials, but they typically do so at amonetary cost or at the expense of cycle life and power density. As anexample of the former, the specific capacitance of hydrous rutheniumoxide has been demonstrated to be higher than 700 F/g, which is fargreater than any known carbon EDLC. However, the cost and lack ofabundant supply of the ruthenium materials prohibit broad commercialuse.

Lower cost, plentiful transition metal oxides such as manganese oxide,nickel oxide and others have been investigated and in some casescommercialized. The specific capacitance of these materials ranges fromapproximately 200 F/g for powder/paste derived thick films (tens ofmicrons to hundreds of microns) to more than 500 F/g for very thinplanar films of less than 100 nanometers. This difference in capacitance(and ultimately energy density) demonstrates the surface nature of thepseudocapacitance charge storage mechanism. However, power density alsois limited by the surface nature of pseudocapacitance as the longer iondiffusion length of thicker material serves to reduce reaction rate. Thepower density of electrodes using these oxides is further limited by thevery low intrinsic electronic conductivity of these materials. Theseoxides also exhibit limited cycle stability and operating voltage rangeslimited to approximately 0.8 volts.

Metal oxide paste-derived electrodes combine the powdered metal oxidewith a binder and conductivity-enhancing carbon similar to the EDLCactivated carbon electrode. The resulting electrode is limited by thecharacteristics noted above and also the lack of surface area readilyaccessible to the electrolyte. The limited pore volume also impedes ionflow to the inner electrode surfaces further limiting thepseudocapacitance reaction rate.

EC devices employing a metal oxide thick film configure their systems insuch a way so as to use the oxide-based electrode as a largely staticenergy storage element creating an offset voltage from which a carbonelectrode only is cycled through charge and discharge. Thisconfiguration is known in the art as an asymmetric EC.

One drawback to this type of asymmetric EC approach is that the cellmust be maintained at a voltage level no less than approximately halfthe rated cell voltage, creating a potential safety hazard. Further,limiting the operating voltage range of the device serves to limit theusable energy as well as limit market applications of the device. Moreideal would be an EC device able to operate over the entire voltagerange while retaining optimally high power density.

Another approach employs a very thin metal oxide planar film. Thisconfiguration yields higher specific capacitance, better reaction rateand is less affected by the low electronic conductivity of the metaloxide. Unfortunately, its limited planar surface area makes itimpractical as an EC electrode.

U.S. Pat. No. 6,339,528 discloses the synthesis of an amorphousmanganese oxide on a non-structural carbon (i.e. carbon powder), whichis then ground to form a paste used with a binder to form an electrode.Others have suggested similarly coating loose, non-structured carbonnanotubes with amorphous manganese oxide subsequently mixing the coatednanotubes with a binder to form an EC electrode. While each of theseapproaches offer improved rate performance resulting from the reducedion diffusion length vs. the simple oxide paste electrodes, they do notresolve the underlying problems associated with electronic conductivityand electrolyte accessibility.

Long et al. (see for example, 20080247118; 20080248192; and 20100176767)have proposed an approach for addressing these shortcomings by applyinga very thin coating of poorly crystalline MnO₂ or iron oxide to a carbonstructure. In doing so, the high capacitance and fast reaction rate ofthe thin film approach is preserved. Further, the 3-dimensional carbonstructure provides a low (electronic) resistance path to the currentcollector and an open porosity providing much improved electrolyte ionaccess to the MnO₂ or iron oxide active material. The synthesis approachsuggested by Long involves the reduction of permanganate or potassiumferrate(VI) on the surface of the carbon. This takes place as thecoating is deposited utilizing the carbon as a sacrificial reductant tosynthesize the oxide. The oxide deposition method suggested by Longresults in a conformal coating of the carbon structure.

Long's approach describes the use of a monolithic carbon structureformed by one of a number of means, but precludes combining of thesemethods as a means of producing a carbon structure more optimally suitedto the intended application and further precludes the use of nitrogendoping to improve conductivity. Long's approach further precludes theuse of carbon metastructures which, while are and provide the benefitsof 3-dimensional carbon structures, they do not comprise a monolithicelectrode. Rather, metastructures form a powder that is used inconjunction with binders and other carbon to form a composite electrodein the traditional way.

While Long's approach does improve many of the shortcomings of the oxideas an EC electrode active material, it is limited to the formation of aMnO₂ or iron oxide only film. Popov et al. with the University of SouthCarolina demonstrated a 10% improvement in operating voltage range and a25% increase in capacity vs. a manganese-only approach. This wasaccomplished by creating an oxide mixture comprising manganese andeither lead oxide or nickel oxide, the latter leading to theseaforementioned improvements. Popov did not utilize a carbon structurebut rather created a mixed oxide powder through Sol-Gel techniques,these powders with a binding agent and conductivity enhancing carbon tocreate a paste electrode.

While the previously discussed improvements in EC technology are highlysignificant, there remains a need in the art for EC devices andtherefore EC electrodes having improved cycle life stability, expandedoperating voltage range and increased the storage capacity while alsoexhibiting improved power density.

Lithium ion secondary batteries operate through theintercalation/de-intercalation of lithium ions into and out of the solidbulk electrode materials. Today's lithium ion electrode materialstypically comprise a graphite-based anode and a transition metal oxide(typically cobalt, nickel or manganese) cathode. During the chargingcycle, electrons are removed from the cathode, which causescharge-compensating lithium ions to be released into and dissolved inthe electrolyte where they migrate towards the anode; while electronsare simultaneously added to the anode causing lithium ions to beinserted into the anode. The opposite occurs during discharge.

Ion insertion (diffusion) into the bulk oxide cathode material takesplace in vacancies present in the oxides. The rate of this process isaffected by the size of the vacancies relative to the ion size, thediffusion length and the accessibility of the electrolyte to the oxide.This rate in turn affects the instantaneous power capability of thebattery.

The fabrication method of these electrodes relies on powdered activematerials formed into a paste including binder material and (electronic)conductivity enhancing carbon. The thickness of these cathodes rangesfrom 30 micrometers for high power (low energy) batteries to 200micrometers for high-energy (low power) versions. Typical oxide powderparticle sizes vary from hundreds of nanometers to a few micrometers indiameter. Lithium ions penetrate these macroscopic cathode structuresthrough the electrolyte and subsequently diffuse as much as a fewmicrons into the bulk oxide particle. The charge-compensating electronsfrom the oxide must then traverse the low-conductivity oxide andelectrolyte to complete the circuit.

Ion diffusion into the solid-state electrode particles inducesmechanical stress on the oxide crystal lattice as it expands toaccommodate the ion insertion. These expansion/contraction cycles causethe eventual breakdown of the oxide limiting device cycle life. It istherefore preferable for the oxide vacancies to be of a size relative tothe ion so as to allow ion diffusion with minimal expansion. For thesame reason, it is also preferable for the diffusion to be as shallow aspossible and to choose ion/oxide systems that exhibit minimal expansion.

While the previously discussed improvements in secondary batterytechnology are highly significant, there remains a need in the art forsecondary battery devices and therefore secondary battery electrodeshaving improved cycle life, shorter recharge time and generallyincreased usable storage capacity at elevated power levels.

SUMMARY OF THE INVENTION

The aforementioned need for improvements in electrode cycle life,operating voltage range and storage capacity for an EC device as well asimprovements in electrode cycle life, reduced recharge times andincreased usable storage capacity at elevated secondary battery powerlevels are provided by the use of an electrode material consisting of ahighly porous carbon structure coated with a film comprising one or moretransition metal oxides. The oxide film is simultaneously synthesizedand deposited thereby forming a conformal oxide coating upon the highlyporous carbon structure.

Such electrode material nanoscopic thin oxide film exhibits highspecific capacity because the oxide active material is more accessibleto the electrolyte ions than is possible with oxide powder-basedapproaches.

The openness of the material provides electrolyte ion access to theoxide film in a way that supports high rate ion transport with minimizedion starvation.

While many factors affect the rate of diffusion of an ion into a metaloxide during intercalation (approximately 10-9 cm²/s for lithium intoLiMnO₂), it is clear such processes occur relatively slowly. Therefore,intercalation reaction rates are faster where diffusion lengths areshorter as is the case in thinner oxide layers. Due to the nanoscopicthin oxide film, the ion diffusion length is as much as two orders ofmagnitude less than that of typical oxide powder based films havingmicron-scale powder grains. The result is an enhanced rate ofintercalation reaction versus powder oxide film approaches.

The carbon substrate exhibits a very high electronic conductivity of20-50 Siemens/cm or more vs. 10⁻⁵ to 10⁻⁸ Siemens/cm for manganeseoxide, depending upon the oxide phase. This reduction in electrodeelectronic resistance of between 5 and 8 orders of magnitude willgreatly improve device power density and reduce heat generated.

The oxide exhibits increased capacity and a greater operating voltagerange with improved stability vs. a single oxide film. For example, inthe case of a nickel/manganese oxide, the partial substitution of thenickel oxide improves structural stability and provides an additionallayered oxide phase improving ion vacancy access vs. a manganese-onlyoxide. Nickel/manganese oxide also has a higher theoretical capacity ata wider electrochemical potential than does manganese oxide alone so theresulting oxide will exhibit increased energy density. The partialsubstitution of a second transition metal oxide such as nickel formanganese in LiMn₂O₄ for example, provides an increase in redoxpotential.

The electrode material not only is suitable as a material for EC andsecondary batteries, but also may be used as fuel cell electrodes,absorption electrodes in water deionization and as electrodes forelectrolysis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of the oxide coating on a porouscarbon structure forming an electrode. 1 represents bare carbonstructure, 2 represents current collector and 3 represents oxide layeron the electrode carbon.

FIG. 2 shows a chart comprising exemplary nickel/manganeseconcentrations.

FIG. 3 is a table showing the oxide mass uptake over a range oftemperatures from ambient to 80° Celsius.

FIG. 4 shows a scanning electron micrograph of a nickel/manganese oxidefilm on a porous supported carbon xerogel structure.

FIG. 5 shows a pseudocapacitive behavior in a cyclic voltammogram forExample 1; poorly crystalline nickel/manganese electrode material at 10mv/S in 1M LiCl.

FIG. 6 shows a constitutional drawing of an electrochemical cell such asan EC or secondary battery. 1 represents the cathode current collector,2 represents the cathode, 3 represents the electrolyte/separator, 4represents the anode and 5 represents the anode current collector.

FIG. 7 shows a more classic insertion redox behavior in a cyclicvoltammogram for Example 3; more crystalline oxide containing spinelphase electrode material at in 2M Li₂SO₄.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed particularly towards electrodematerials created by forming a thin coating of metal oxide on a highlyporous carbon “metastructure”, which is a sub-unit of an electrode andis in the form of a micro-scale powder with nano-scale features;essentially an extrapolation of nano-scale carbon to the micron scale.One purpose of the metastructure form is to provide the surface accessand diffusion benefits of nanomaterials with improved electronictransport of a micro or macro-scale structure. Another purpose of themetastructure is to provide materials with these aforementionedcharacteristics in a size and form that is similar to typical capacitorand battery materials used by the industry already permitting the use ofcurrent manufacturing techniques that are compatible with currentmanufacturing equipment in ways nanoscale materials in and of themselvesmay not be. These electrode materials are for use in electrochemicalenergy storage devices including electrochemical capacitors andsecondary batteries when combined with additional materials such asbinder and conductivity-enhancing carbon black. Such electrochemicalcapacitor or secondary battery includes for example an electrolyte, anelectronically insulating but ionically conductive separator film, apair of electrodes separated by said separator and electrolyte, eachelectrode physically attached and electronically connected to a currentcollector, wherein at least one of said electrodes comprise theelectrode material comprising a porous carbon structure with a conformalsurface coating of metal oxide as described herein. The electrolytecomprises salts of alkali metal in an aqueous solvent, in a non-aqueoussolvent, in a polymer, as an ionic liquid or any combination thereof.The second electrode, if not an electrode material as defined herein, isselected from a group consisting of one or more metal oxides; a metalphosphate, a metal carbide; a metal nitride; a composite carbonaceouspaste comprising powder of one or more of activated carbon or carbonnanofibers or carbon nanotubes or graphene or any combination thereof,with binder and conductivity enhancing carbon; a composite carbonaceouspaste comprising graphitic carbon powder, hard carbon powder, metaloxide/carbon composites, silicon/carbon composites, or any combinationthereof with binder and conductivity enhancing carbon; or a porousactivated carbon structure. The current collector is selected from agroup consisting of metal foil, metal mesh, electrically conductivepolymer composites, expanded metal, or combinations thereof.

Hereinafter, various embodiments of the present invention will beexplained in more detail with reference to the accompanying figures;however, it is understood that the present invention should not belimited to the following preferred embodiments and such presentinvention may be practiced in ways other than those specificallydescribed herein.

The electrode material comprises a porous carbon meta-structure with aconformal surface coating of metal oxide wherein said coating isproduced by an oxidation/reduction reaction occurring between the metalsalt contained in an aqueous precursor solution and the surface of saidporous carbon when said porous carbon is infiltrated with said precursorsolution; wherein transition metal species contained in said precursorsolution are reduced on the surface of the carbon and co-deposited inoxide form upon the carbon; wherein said aqueous precursor solution ismaintained at a temperature above about 20° C. and below about 250° C.during said infiltration; wherein an autoclave is the reaction vesselwhen synthesis temperatures above about 100° C. are used; wherein saidinfiltration is accomplished by immersion and equilibration of saidcarbon structure in a bath of said aqueous metal salt precursor solutionor by application of pressure spray consisting of said aqueous metalsalt precursor solution upon said carbon meta-structure; wherein thesolvent of said aqueous metal salt precursor solution shall contain oneor more of purified water, an organic solvent such as an alcohol, a pHbuffer, additional cation salts or any combination thereof; wherein saidaqueous metal salt precursor solution shall comprise one or more saltsof metals selected from a group consisting of manganese, nickel, cobalt,iron, aluminum, chromium, molybdenum, rhodium, iridium, osmium, rhenium,vanadium, tungsten, tantalum, palladium, lead, tin, titanium, copper,zinc, niobium and lithium; wherein the electrode material is used asprepared or the counter ions incorporated in the oxide coating areexchanged for other cations or protons; wherein the formed electrodematerial is used as-prepared or wherein the formed electrode material isheated subsequent to formation of the oxide coating, such heating tooccur as hydrothermal processing at temperatures above about 70° C. andbelow about 250° C. in an autoclave or with the use of microwaveradiation or at temperatures above about 70° C. and below about 1000° C.in inert atmosphere or in oxidizing atmosphere or in reducing atmosphereor any combination thereof. In one embodiment, said oxidation/reductionreaction between said porous carbon structure and said aqueous metalsalt precursor solution occurs while the reactants are exposed tomicrowave energy. In one embodiment, said aqueous precursor solutionshall comprise ultrapure water, or a buffer solution with or withoutorganic co-solvent or additional cations, further comprising one or moremetal salt in the form of M(NO_(y))_(z)xH₂O, MCl_(y)xH₂O, MF_(y),MI_(y), MBr_(x), (MCl_(y))_(z)xH₂O, M(ClO_(y))_(z)xH₂O, MF_(y),M_(y)(SO_(z))_(w), MSO_(y)xH₂O, M_(y)P, MPO_(y)xH₂O, M(OCH_(y))_(z),MOSO_(y)xH₂O, M(C_(y)O_(z))xH₂O, where x is a value greater than orequal to 0 and less than or equal to 12 and y is a value greater than orequal to 0 and less than or equal to 4 and z is a value greater than orequal to 0 and less than or equal to 4 and w is a value greater than orequal to 0 and less than or equal to 4, and M is selected from a groupconsisting of manganese, nickel, cobalt, iron, aluminum, chromium,molybdenum, rhodium, iridium, osmium, rhenium, vanadium, tungsten,tantalum, palladium, lead, tin, titanium, copper, zinc, niobium andlithium; or NaMnO₄, KMnO₄, LiMnO₄, K₂FeO₄; or titanium(III) chloridetetrahydrofuran complex (1:3), titanium diisopropoxidebis(acetylacetonate), titanium(IV) isoproprxide, titanium(IV)(triethanolaminato) isoproprxide, titanium(IV) bis(ammoniumlactato)dihydroxide, titanium(IV) butoxide, titanium(IV) ethoxide,titanium(IV) oxyacetylacetonate, titanium(IV) phthalocyanine dichloride,titanium(IV) propoxide, titanium(IV) sulfide, titanium(IV)tert-butoxide, titanium(IV)2-ethylhexyloxide, K₂TiF₆,FeSO₄NH₃CH₂CH₂NH₃SO₄4H₂O, Iron(II) acetate, Iron(II) acetylacetonate,ammonium iron(III) oxalate trihydrate, Iron(III) citrate, NaNO₃, KNO₃,LiNO₃, Na₂SO₄, K₂SO₄, Li₂SO₄, NaOH, KOH, LiOH. At synthesis temperaturesabove about 100° C., an autoclave is used.

The metal oxide coating may comprise water, ions and shall contain oneor more metal oxides selected from a group consisting of oxides ofmanganese, nickel, cobalt, iron, aluminum, chromium, molybdenum,rhodium, iridium, osmium, rhenium, vanadium, tungsten, tantalum,palladium, lead, tin, titanium, copper, zinc, niobium and lithium.

In one embodiment, the porous carbon meta-structure is composed of apolymer-derived carbon xerogel formed in the presence of additionalcomponent material selected from a group consisting of carbonmicrofibers, carbon nanofibers, carbon nanotubes, graphite, graphene,carbon black, activated carbon or any combination thereof. The porouscarbon structure may be formed with or without templating agents, may beactivated or not activated and may be doped with nitrogen or un-doped.

Nitrogen doping of carbon materials is used as a method to increaseelectronic conductivity by modifying the partially p-type carbon to amore n-type material, thereby increasing electron concentration in theconduction band. In one embodiment, ammonia is used as a nitrogensource. In other embodiments, urea or melamine is used as a nitrogensource.

Coated meta-structure electrode material may be used as-synthesized orthe counter ions may be fully exchanged or partially exchanged for adifferent ion species or for protons. Coated electrode materials may beheated subsequent to formation of the nanoscale oxide coating; suchheating to occur as hydrothermal processing at temperatures above about70° C. and below about 250° C. in an autoclave or with the use ofmicrowave radiation or at temperatures above about 70° C. and belowabout 1000° C. in inert atmosphere or in oxidizing atmosphere or inreducing atmosphere or any combination thereof. Such ion exchange andheating techniques represent some of the synthetic controls that can beused to create oxide phases suitable to specific applications. Forexample, xMO₂/C where C is the carbon structure, x is the cation and Mis a poorly crystalline birnessite or other phase manganese and/or otheroxide formed on the carbon using the synthesis herein at ambientconditions provides a pseudocapacitance-type reaction suitable ascathode material for aqueous electrochemical capacitor applications. Inanother example, a spinel-type oxide phase/carbon is created by cationexchange for lithium followed by heat treatments following synthesis ofthe aforementioned poorly crystalline oxide film. The spinel LiM₂O₄/C isformed where M may be manganese with or without dopants or partialsubstitutions with elements such as nickel, may be used as cathodematerial suitable for aqueous or non-aqueous electrochemical capacitorapplications, or as cathode material for secondary lithium-ion batteryapplications.

Other oxide coatings for carbon structures are contemplated such asLi₄M₅O₁₂/C, LiMO₂/C or Li_(28+y)M₂₀O₄₈/C where 0<y<8 and where M may betitanium with or without niobium and/or tantalum and/or vanadium asdopants or partial substitutions as M₂O₇/C or independent oxides asM₂O₅/C. In these cases, the oxide/carbon material may be used as anodematerial in non-aqueous electrochemical capacitor applications, or asanode material for secondary lithium-ion battery applications.

Another example of metal/oxide coatings for carbon structurescontemplated herein include M₃O₄/C where M may be manganese and/or ironand/or cobalt. In this case, M is cycled between low-valence oxide andmetallic states, and may be used as anode material for use in secondarybattery applications such as lithium ion. These materials may besynthesized, for example, using permanganates alone and/or nitrates ofmanganese and/or cobalt. In one embodiment, the permanganate is used asa reducing agent and a source of manganese; cobalt nitrate, for example,may be optionally used with the permanganate as an additional ionsource. In the case wherein permanganate is not used, (as in the cobaltcase or manganese oxide not using permanganate route) a precursor saltsuch as a nitrate may be used in an aqueous solution with reducingagents such as an alcohol and/or ammonia at ambient or othertemperatures. In the case of Fe₃O₄/C, iron salts such as potassiumferrate and/or iron(III) chloride hexahydrate may be used as precursormaterials. In all cases, the MOx/C materials are subsequently heated totemperatures ranging from about 100° C. to about 250° C. as hydrothermalprocessing in an autoclave or from about 250° C. to about 600° C. ininert atmosphere for between 1 and 24 hours. In certain cases,subsequent heating to temperatures ranging from about 100° C. to about300° C. in air may be required to obtain the desired oxygenstoichiometry. Also, in some cases, lithium may be used as the counterion prior to heating for the purpose of assisting in templating thedesired oxide phase and/or providing a source of lithium that may beappropriate in a lithium ion device. In some cases, the counter-ions maybe exchanged for protons prior to or following heating.

Example I

Fabrication of electrode; birnessite manganese/nickel oxide film oncarbon nanofoam.

An electrode material as illustrated in FIG. 1 was formed by immersing acarbon structure for a controlled period of time in a solutioncomprising permanganate and nickel salts in a controlled ratio dissolvedin ultra-pure water/pH buffer at a controlled pH and temperature. Themanganese and nickel from the aqueous permanganate/nickel precursorsolution are reduced on the surface of the carbon and co-deposited uponthe carbon forming an insoluble oxide film.

Carbon paper and aerogel (“nanofoam”) was purchased from a commercialsource (Marketech International Inc.) with an approximate thickness of170 micrometers. Carbon nanofoam paper was cut into pieces ofapproximately 1 centimeter by 1 centimeter and then soaked and vacuumsaturated in purified water.

In this exemplary embodiment, the aqueous metal salt precursor solutioncomprised manganese/nickel mixture ratios as follows: 4:1, 2:1, 1:1, 1:2and 1:4. FIG. 2 shows the specific capacities for the first three ratiosand one sample of manganese only for comparison purposes. In thesecases, the mixture concentrations of nickel (II) nitrate hexahydrate(Ni(NO₃)₂6H₂O) were normalized to 0.1 M sodium permanganate (NaMnO₄,other counter-ion sources may be substituted for sodium (Na) such aspotassium (K) or lithium (Li)) and combined with purified water/pHbuffer solution of 0.1M NaH₂PO₄ and 0.1M NaOH for neutral pH filmsynthesis. Another experiment was carried out at an elevated pH of 12using a buffer solution of 0.05M Na₂HPO₄ and 0.1M NaOH.

The wetted carbon nanofoam was then immersed in the precursor solutions,vacuum equilibrated and left immersed for a period of time ranging fromapproximately 15 minutes to 20 hours. These synthesis processes werecarried out at room temperature.

The resulting electrode materials were removed from the precursorsolution, rinsed with purified water and dried in a nitrogen environmentat 50° C. for 20 hours and again under vacuum at room temperature for anadditional 12 hours.

Example II

Characterization of electrode material; manganese/nickel oxide film oncarbon nanofoam

The resulting electrode structure is shown in FIG. 4 scanning electronicmicrograph image. This image clearly shows the material feature scale,the conformal oxide coating and the absence of pore occlusion.

The table in FIG. 2 shows a 32% increase in capacitance of the 4:1manganese/nickel oxide (131.4 F/g) vs. the manganese only material(99.27 F/g) in 1M LiCl electrolyte. Subsequent experiments have yieldedcapacitances of 180 F/g for the 4:1 manganese/nickel oxide in thiselectrolyte and approximately 200 F/g in other electrolytes such aspotassium hydroxide (KOH). FIG. 5 shows cyclic voltammetry data of a 4:1manganese/nickel oxide material with capacitance of approximately 180F/g in 1M LiCl electrolyte.

Example III

Fabrication of electrode; nanoscale oxide film comprising spinelmanganese doped with nickel on carbon nanofiber/microfiber supportedcarbon xerogel structure.

An electrode material is formed as in EXAMPLE I, using a precursor ratioof about 0.99:0.01 manganese:nickel.

Prior to drying, counter ions are exchanged for lithium ions byimmersion of the formed electrode in an aqueous solution bearing lithiumions such as lithium nitrate, lithium sulfate or lithium hydroxide, forexample. In this exemplary embodiment, lithium nitrate was used. Suchimmersion is carried out first under vacuum equilibration, then at roomtemperature or elevated temperature or under microwave heating, forexample. In this exemplary embodiment, about 30° C. for about 2-4 hourswas used.

The material was subsequently heated to about 300-350° C. under nitrogenatmosphere for about 1-2 hours, followed by heating at about 200-220° C.in air for about 3-6 hours.

Example IV

Characterization by cyclic voltammetry of electrode; nanoscale oxidefilm comprising spinel manganese doped with nickel on carbonnanofiber/microfiber supported carbon xerogel structure.

FIG. 7 shows cyclic voltammetry data of nickel doped manganesespinel/carbon material with average capacitance of approximately 200 F/gbetween about 600 mv and 900 mv vs. Ag/AgCl in 2M Li₂SO₄ electrolyte.Noteworthy are the redox peaks not present in the more disorderedbimessite/nickel oxide of examples 1 and 2, indicating the presence ofspinel phase in the doped and heated oxide layer.

Example V

Fabrication of electrode; nanoscale oxide film comprising M₃O₄ where Mis manganese doped with cobalt on carbon nanofiber/microfiber supportedcarbon xerogel structure.

An electrode material is formed as in EXAMPLE I, using a precursor ratioof about 0.99:0.01 manganese:cobalt.

Prior to drying, counter ions are exchanged for lithium ions byimmersion of the formed electrode in an aqueous solution bearing lithiumions such as lithium nitrate, lithium sulfate or lithium hydroxide, forexample. In this exemplary embodiment, lithium nitrate was used. Suchimmersion is carried out first under vacuum equilibration, then at roomtemperature or elevated temperature or under microwave heating, forexample. In this exemplary embodiment, 30° C. for 4 hours was used.

The material was subsequently heated to about 300-350° C. under nitrogenatmosphere for about 1-2 hours followed by removal of ions by protonexchange with dilute acid, subsequent rinsing and drying.

Although embodiments of the invention have been described, it isunderstood that the present invention should not be limited to thoseembodiments, but various changes and modifications can be made by oneskilled in the art within the spirit and scope of the invention ashereinafter claimed.

1. An electrode material comprising: a conformal surface coating ofmetal oxide, said metal oxide coating comprising one or more ofmanganese, nickel, cobalt, iron, aluminum, chromium, molybdenum,rhodium, iridium, osmium, rhenium, vanadium, tungsten, tantalum,palladium, lead, tin, titanium, copper, zinc, niobium or lithium,wherein the average thickness of said metal oxide coating is greaterthan about 5 nanometers and less than about 1000 nanometers, said metaloxide coating consisting of a crystalline phase, a partially crystallinephase, an amorphous phase or any combination thereof; and a porouscarbon meta-structure powder comprising a carbon xerogel formed with oneor more carbon microfibers, carbon nanofibers, carbon nanotubes,graphite, graphene, carbon black or activated carbon, said porous carbonstructure comprising pores of above average pore size and pores of belowaverage pore size, wherein said pores of above average pore size have anaverage diameter larger than about two nanometers and wherein pores ofbelow average pore size are smaller than about five micrometers; whereinthe average particle size of said porous carbon meta-structure isgreater than about 100 nanometers and smaller than about 100 micrometersin its largest dimension.
 2. The material of claim 1, wherein the porouscarbon meta-structure is formed with use of a template.
 3. The materialof claim 1, wherein the porous carbon meta-structure is formed with useof a nitrogen dopant.
 4. The oxide coating of claim 1 having the generalformula M₂O₇, M₂O₅, M₃O₄, MO₂, LiMO₂, LiM₂O₄, Li₄M₅O₁₂, orLi_(28+y)M₂₀O₄₈, wherein y is greater than or equal to 0 and less thanor equal to 8, and wherein M is one or more of manganese, nickel,cobalt, iron, aluminum, chromium, molybdenum, rhodium, iridium, osmium,rhenium, vanadium, tungsten, tantalum, palladium, lead, tin, titanium,copper, zinc, niobium or lithium.
 5. A method for producing an electrodematerial comprising: reacting a metal salt with a surface of a porouscarbon structure, wherein said reacting is an oxidation/reductionreaction, wherein said metal salt is contained in an aqueous precursorsolution, said precursor solution further comprising ionic species, andwherein said porous carbon structure is infiltrated with said precursorsolution; reducing said ionic species on the surface of said carbonmeta-structure and co-depositing in oxide form upon said carbonstructure, wherein said aqueous precursor solution is maintained at atemperature above about 20° C. and below about 250° C. during saidinfiltration; said infiltration comprising: a/ immersing andequilibrating said carbon structure in a bath of said aqueous metal saltprecursor solution, or b/ applying pressure spray consisting of saidaqueous metal salt precursor solution upon said carbon structure; saidaqueous metal salt precursor solution comprising a solvent, said solventcomprising at least one of purified water, organic solvent, pH buffer,cation salts or any combination thereof, said metal salt of said aqueousmetal salt precursor solution selected from the group of metalsconsisting of manganese, nickel, cobalt, iron, aluminum, chromium,molybdenum, rhodium, iridium, osmium, rhenium, vanadium, tungsten,tantalum, palladium, lead, tin, titanium, copper, zinc, niobium, lithiumand combinations thereof; wherein said electrode material is a porouscarbon meta-structure with a conformal surface coating of metal oxide.6. The method of claim 5, said aqueous precursor solution comprisingultrapure water, or a buffer solution further comprising one or moremetal salt(s) in the form of M(NO_(y))_(z)xH₂O, MCl_(y)xH₂O, MF_(y),MI_(x), MBr_(y), (MCl_(y))_(z)xH₂O, M(ClO_(y))_(z)xH₂O, MF_(y),M_(y)(SO_(z))_(w), MSO_(z)xH₂O, M_(y)P, MPO_(y)xH₂O, M(OCH_(y))_(z),MOSO_(y)xH₂O, or M(C_(y)O_(z))xH₂O, where x is a value greater than orequal to 0 and less than or equal to 12, y is a value greater than orequal to 0 and less than or equal to 4, z is a value greater than orequal to 0 and less than or equal to 4, w is a value greater than orequal to 0 and less than or equal to 4, and M is selected from a groupconsisting of manganese, nickel, cobalt, iron, aluminum, chromium,molybdenum, rhodium, iridium, osmium, rhenium, vanadium, tungsten,tantalum, palladium, lead, tin, titanium, copper, zinc, niobium,lithium, NaMnO₄, KMnO₄, LiMnO₄, K₂FeO₄, titanium(III) chloridetetrahydrofuran complex (1:3), titanium diisopropoxide bis(acetylacetonate), titanium(IV) isoproprxide, titanium(IV)(triethanolaminato) isoproprxide, titanium(IV) bis(ammoniumlactato)dihydroxide, titanium(IV) butoxide, titanium(IV) ethoxide,titanium(IV) oxyacetylacetonate, titanium(IV) phthalocyanine dichloride,titanium(IV) propoxide, titanium(IV) sulfide, titanium(IV)tert-butoxide, titanium(IV) 2-ethylhexyloxide, K₂TiF₆,FeSO₄NH₃CH₂CH₂NH₃SO₄4H₂O, Iron(II) acetate, Iron(II) acetylacetonate,ammonium iron(III) oxalate trihydrate, Iron(III) citrate, NaNO₃, KNO₃,LiNO₃, Na₂SO₄, K₂SO₄, Li₂SO₄, NaOH, KOH, and LiOH; wherein said aqueousprecursor solution contains no co-solvent, or said aqueous precursorsolution contains an organic co-solvent.
 7. The method of claim 5wherein said oxidation/reduction reaction between said porous carbonstructure and said aqueous metal salt precursor solution occurs whilethe reactants are exposed to microwave energy.
 8. A capacitor cellhaving at least one electrode of the material of claim 1, furthercomprising: an electrolyte; an electronically insulating but ionicallyconductive separator film; a second electrode, said second electrodecomprising: a composite comprising the material of claim 1, a binder,and at least one of activated carbon, hard carbon, carbon nanotubes,carbon nanofibers, graphene or graphite, a porous activated carbonstructure, or a composite comprising a binder and a conductivityenhancing carbon and powder, said conductivity enhancing carbon andpowder comprising one or more of a metal oxide, a metal carbide, a metalnitride, a metal phosphate, activated carbon, hard carbon, carbonnanotubes, graphene or graphite; and a current collector; said oneelectrode and said second electrode being separated by said separatorfilm, each said electrode physically attached and electronicallyconnected to said current collector; said electrolyte comprising saltsof alkali metal in at least one of an aqueous solvent, a non-aqueoussolvent, a polymer, an ionic liquid or any combination thereof; whereinsaid current collector is selected from a group consisting of metalfoil, metal mesh, electrically conductive polymer composites, expandedmetal or any combination thereof; wherein said optional carbon structureor carbon powder is used with or without nitrogen doping.
 9. A secondarybattery cell comprising: an electrolyte; an electronically insulatingbut ionically conductive separator film; a pair of electrodes, at leastone of said pair of electrodes comprising: a composite comprising thematerial of claim 4, a binder, and at least one of activated carbon,hard carbon, carbon nanotubes, carbon nanofibers, graphene or graphite,wherein said pair of electrodes is separated by said separator film; acurrent collector, each electrode physically attached and electronicallyconnected to said current collector; and electrolyte; said electrolytecomprising salts of alkali metal in at least one of an aqueous solvent,a non-aqueous solvent, a polymer, an ionic liquid or any combinationthereof; wherein said current collector is selected from a groupconsisting of metal foil, metal mesh, electrically conductive polymercomposites, expanded metal or any combination thereof.
 10. The method ofclaim 5, wherein cation(s) of said ionic species is incorporated withinsaid metal oxide, further comprising exchanging said cation(s)incorporated within said oxide coating for other cations or protons. 11.The method of claim 5, further comprising heating said electrodematerial subsequent to formation of said metal oxide coating, whereinsaid heating is above about 70° C. and below about 250° C. ashydrothermal processing in an autoclave or wherein said heating is aboveabout 70° C. and below about 1000° in inert atmosphere, oxidizing, orreducing atmosphere or any combination thereof.